Fig. 1. Oxygen consumption reported by myoglobin deoxygenation monitored at 625 nm is determined in mitochondrial suspensions containing myoglobin and limiting amounts of ADP. ADP is exhausted at the arrows. Myoglobin, 500 µmol l^–1. A, 500 µmol l^–1 ADP; B, 1000 µmol l^–1 ADP; C, 2000 µmol l^–1 ADP. Initial rates are 200 mol O₂ (mol cytochrome aa₃)^–1 min^–1. P/O ratios are: 2.3, 2.4 and 2.4, respectively, with respiratory control indices of 3.1–3.4. The P/O ratio of this preparation of mitochondria, determined polarographically, was 2.4 with RCI>6. This shows that mitochondrial oxygen uptake is tightly coupled to phosphorylation when the mitochondria are consuming myoglobin-bound oxygen. AU, absorbance units.